[GSBN] Sheathing and straw bales proposed

Derek Stearns Roff derek at unm.edu
Sat Jul 19 13:29:01 CDT 2014

Hi, Ian,

The Vermont COStraw Nostraw has made a good case for blown in dense-pack cellulose in direct contact with the outside of the bales.  (The COStraw Nostraw is an inner circle of the strawbale mafia, which insists on putting Cellulose Over Straw, or using no straw at all.)  The dense-pack cellulose can compensate for the fire, air movement/convection/thermal performance aiding, and vermin impeding qualities that the exterior plaster normally adds to the wall.  I’m skeptical that batt insulation could be an effective substitute.  Gaps are universal with batt insulation, and good installation just means fewer and smaller gaps.  Given the irregular shape of a strawbale wall profile, I expect there would be too many gaps between the straw and the batts, and the presence or absence of sufficient tight contact would be impossible to inspect and verify.  The idea that batts will “settle tight against the bales” seems very unlikely to reach the desired levels of contact between the two materials.  Maybe our resident COStraw practitioner will offer his opinion.  If your batt insulation is really settling, doesn’t that imply gaps at the top of the wall?  

I’ll add the usual warning, that lime over clay needs to be tested with the specific lime and the specific clay you will use, since some combinations refuse to collaborate.  

On the moisture question, Joe Lstiburek of Building Science Corporation lists both OSB and plywood as “vapor semi-permeable”, and "lightweight asphalt impregnated building papers (#15 building paper)” as being vapor permeable.  My memory of John Straube’s presentations at your Colorado ISBC is that plywood has variable permeability, which increases as moisture levels increase.  That advantage, plus its greater resistance to mold and moisture, and its fewer objectionable chemicals, leads to my preferring plywood over OSB.  It’s worth doing the calculations, but I think that in Boulder’s climate, you will be OK on the permeability question.  


I look forward to seeing your project during a future visit to Boulder.  

By the way, there is a very useful section on moisture for strawbale buildings in "Design of Straw Bale Buildings: The State of the Art", by Bruce King, et al.  Parts of this wonderful book are now available as a free PDF download at the BuildWell Library, on (you guessed it) Bruce King’s website, while some chapters can be downloaded individually at low costs.  An excerpt of the moisture chapter (written by John Straube) is available free at the second link below, and the whole chapter can be downloaded from the third link for $6.  The entire book can be downloaded from the same link for $20.  



On Jul 18, 2014, at 10:48 PM, Ian Smith <ian at lopezsmolensengineers.com> wrote:

> Hello GSBN,
> This is my first time posting here... hope I'm doing this right.  Quick intro of me:  I am a structural engineer that started working on straw bale projects in 2003, with Jeff Ruppert.  Jeff is over in Paonia now, and I'm still in Boulder.  Pretty sure I'm also still the Treasurer of COSBA (Colorado Straw Bale Association), although we're on some sort of extended vacation after hosting the ISBC in 2012.
> Anyway, my wife and I have recently bought a small house in town and we're hoping to do a strawbale addition on it.  The addition will be rectangular with a gable roof.  We're thinking of building a wall system that would be relatively experimental for us here, although I understand that others are doing similar things in Vermont, and maybe Europe...?
> I just wanted describe our proposed wall assembly and see if anyone has any comments, etc.
> The assembly would be, from interior to exterior:
> "Conventional" 3-coat interior clay plaster with lime finish coat(s) applied directly to straw bales, stacked on-edge.
> 2x4 (4x9 cm) wood studs at standard spacing with blown-in cellulose or batt insulation.
> 1/2" (1.25 cm) Structural wood sheathing where needed (structurally), and fiberboard elsewhere.
> Exterior finish A) On walls under eaves (south and north) - double building paper and 2-coat lime plaster
> Exterior finish B) On walls exposed more to weather - weather resistant barrier, vertical wood nailers, and horizontal wood (or similar) rainscreen.
> This wall assembly potentially allows for the complete construction of the exterior walls - with finish and windows installed - and roof, before the straw bales and stud insulation are installed.  Therefore, installation of the straw bales can (potentially) occur at any time of year or in any weather, since the exterior of the building is 100% complete.  No need for temporary bracing.  It also provides some savings in labor since the need for "normal" exterior plaster is greatly reduced (although additional, expensive finish materials are needed).
> Yes, the exterior side of the bales will have no plaster.  Although, it could be possible to do a slip (thin) coat on each bale while stacking.  The batts or blown cellulose between studs would (theoretically) settle tight against the bales, mitigating the potential for air movement there.  I assume the insulation would also contain a fire retardant.  Differential settlement of the bales would be mitigated by tying the bales to the studs, and providing proper support for the interior plaster at the foundation level.
> Most of my concerns with this proposed assembly are focused on building science and moisture issues.  I think I need to use the fiberboard sheathing wherever possible to achieve the highest (overall) permeability possible, but I'm not fully sure...  Also, are there any permeability issues with the double building paper and lime plaster when applied directly to the sheathing?  Should I maybe install a drainage plane behind the plaster?
> Let me know if you think of anything else.  Thanks very much,  (and Dirk, congratulations on the big SB building!)
> Ian Smith, P.E.
> Boulder, CO, USA
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Derek Roff
derek at unm.edu

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